This invention relates to optical recording media of the type wherein information is recorded and reproduced with the use of heat and light of laser.
Among optical recording media are optical magnetic recording media or optical magnetic memories. There are well known a number of materials for the recording layer of such optical magnetic recording media, for example, MnBi, MnAlGe, MnSb, MnCuBi, GdFe, TbFe, GdCo, PtCo, TbCo, TbFeCo, GdFeCo, TbFeO.sub.3, GdIG (gadolinium iron garnet), GdTbFe, GdTbFeCoBi, CoFe.sub.2 O.sub.4, etc. These materials are deposited on transparent substrates of plastic material or glass as a thin film by any suitable thin-film forming techniques such as vacuum deposition or sputtering. The feature common to these optical magnetic recording thin film layers is that the axis of easy magnetization is perpendicular to the film surface. Other features are great Kerr and Farady effects.
By taking advantage of these features, the following optical magnetic recording process is contemplated for such optical magnetic recording thin films. At the outset, the entire film is rendered "0", that is, uniformly magnetized or erased. A laser beam is applied to the film at the site where it is desired to record "1". The temperature of that region of the film exposed to the beam is increased, and coercive force Hc approaches 0 when the temperature approaches and then exceeds the Curie point. When the temperature is allowed to return to room temperature after extinction of the laser beam, the magnetization is reversed by the energy of diamagnetic field. Alternatively, the temperature is allowed to return to room temperature with an external magnetic field being applied in an opposite direction to that at the initial during exposure to the laser beam, and then magnetization is reversed. There is recorded a signal "1". The remaining portion of the film where no laser beam is incident remains "0" because the initial state is "0".
The recorded data in the optical magnetic memory is read out by similarly using a laser beam to detect the magnetooptic effect, that is, the rotation of the polarization plane of reflected light with respect to the incident laser beam due to the reversal of magnetization.
Requirements imposed on such optical magnetic recording media are:
(1) that the compensation point is relatively low, and particularly near room temperature, PA0 (2) that the Curie point is an adequate temperature, PA0 (3) that noise-inducing defects such as grain boundary is relatively small, and PA0 (4) that a magnetically and mechanically uniform film is attained over a relatively large area without resorting to a high temperature or long term film formation process.
In the light of these requirements, a great attention is recently drawn to amorphous perpendicular magnetizable thin films of rare earth element-transition metal among the above-mentioned materials. Optical magnetic recording media having such amorphous perpendicular magnetizable thin films of rare earth element-transition metal, however, require a rather complicated film formation process because the thin films contain a major proportion of rare earth elements, and are thus expensive to manufacture. There are additional disadvantages including an unstable film formation process, inconsistent electromagnetic properties, and low reliability.
Thus there is the need for solving these problems and it is particularly desired to make the perpendicular magnetizable thin films thinner and to reduce the amount of rare earth elements used.
The role rare earth elements play in thin films is to amorphitize the films and to improve Kerr rotation angle or Faraday rotation angle. These functions are essential to optical magnetic recording media and difficult to substitute some other means therefor. In optical magnetic recording media of the prior art structure, it was impossible to thin the recording layer thin film, to mitigate the role of rare earth elements, to achieve a stable and less expensive film formation step, and to realize improved reliability and electromagnetic properties while keeping the necessary functions unchanged. For example, the provision of various protective layers on the recording layer thin films has been proposed. Such approaches did not make essential access to the great role of rare earth elements in the media. These protective layers were insufficiently humidity proof to prevent rare earth elements from being preferentially attacked by oxygen and water in a high humidity atmosphere, resulting in deteriorated recording and reproducing properties.